September 12, 2011

Unique prospects of graphene-based THz modulators

The modulation depth of 2-D electron gas (2DEG) based THz modulators using AlGaAs/GaAs heterostructures with metal gates is inherently limited to less than 30%. The metal gate not only attenuates the THz signal (over 90%) but also severely degrades the modulation depth. The metal losses can be significantly reduced with an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high mobility of holes that is comparable to electron mobility in conventional semiconductors. The hole conductivity in graphene can be electrostatically tuned in the graphene-2DEG parallel capacitor configuration, thus more efficiently tuning the THz transmission. In this work, we show that it is possible to achieve a modulation depth of over 90% while simultaneously minimizing signal attenuation to less than 5% by tuning the Fermi level at the Dirac point in graphene.

(a) Operating principle of a 2DEG-based THz modulator. THz transmission through a
conducting media (2DEG) is tuned by a voltage applied between the top gate and the 2DEG. THz transmission is high with low 2DEG densities, and low with high 2DEG densities due to enhanced absorption and reflection. (b) Layer structures of traditional metal-gate/2DEG and proposed graphene/2DEG and graphene/graphene THz modulators. Shown in the box are the schematic energy band diagrams of a graphene/insulator/graphene modulator that promises near zero beam attenuation and unity modulation depth. When the Fermi level is at the Dirac point of both the top and bottom graphene layers, THz transmission approaches unity; when electron and hole sheets of charges are formed in the top and bottom graphene layers, THz transmission nears zero.

In conclusion, we have presented an analytical study on the current limits of performance of 2DEGbased THz modulators, and how incorporating graphene as the ‘tunable metal’ gate holds promise for significant improvements in the performance. In the previously proposed metal/AlGaAs/2DEG/GaAs structures, the maximum modulation depth is inherently limited to be less than 30% by the adverse effect of the highly conductive gate metal as well as the maximum achievable 2DEG sheet conductivity. A single layer graphene can be nearly transparent when its Fermi level is tuned at the Dirac point and block almost 100% THz beam when tuning to its maximum conductivity, which is extraordinary compared to any other 2DEG system. By adopting graphene in 2DEG THz modulators, negligibly low beam attenuation and near unity modulation depth are achievable, offering advantages including RT, broadband, and polarization-independent operation.